1. Field of the Invention
This invention relates to a Ni-Fe magnetic alloy having excellent magnetic characteristics and productivity and to a method for producing thereof.
2. Description of the Related Arts
Ni-Fe alloys corresponding to PC (referred to simply as "PC Permalloys" hereafter) defined in JIS (Japanese Industrial Standards) C2531 have widely been used as casings and cores of magnetic heads, magnet cores of various types of transformers, magnetic insulations, etc.
However, ingots of PC Permalloy is inferior in hot workability, and when they are subjected to slabbing, the prepared slabs unavoidably suffer surface defects owing to the reason described later. The hot workability of PC Permalloy varies with the Ni content, and the higher the content of Ni becomes, the more the hot workability degrades. Consequently, the hot workability of an ingot of PC Permalloy containing approximately 80 wt. % Ni is significantly inferior to that of Ni-Fe alloy ingot containing 35 to 45 wt. % of Ni. As a result, in a prior art, slabbing could not be applied for a PC Permalloy ingot to obtain a slab having less surface defects such as edge cracks, or having an excellent surface property, so the forging method was forcefully applied. The reason why the forging method presents a slab having less surface defects is that the method applies mainly compressive force compared with the slabbing in which method multi-axial stress and shearing stress work to an ingot. Different from slabbing method, the forging method gives a poor hot working efficiency, and still it can not drastically reduce the generation of slab surface defect. Accordingly, the forging method also needs a step to remove the slab surface defects, which raises a problem of extra labor and time.
When an ingot of poor hot workability, including PC Permalloy, is subjected to slabbing to form a slab, the obtained slab likely has a lot of surface defects. The reason for the phenomenon is that an ingot experienced slabbing deforms at 1.times.1 s.sup.-1 or more strain rate and that the temperature at the edge and surface layer at that time is lower than the temperature at the central region of the ingot to become as low as 900.degree. C. The strain rate is represented by strain which occurs for a second as an unit time. As a result, an ingot which has such a temperature gradient within the body induces surface defects such as edge cracks when it is deformed by slabbing.
Particularly when an ingot of PC Pennalloy which has poor hot workability is subjected to slabbing, impurity elements begin to segregate at the grain boundaries of austanitic phase during the temperature reduction period of the ingot and to bring the grain boundaries to an embrittlement state, which markedly reduces the ductility at a temperature range of 950.degree. to 1000.degree. C. of the ingot, which then induces lots of defects on the slab surface.
This type of hot workability problems occur also during the production of press shapes by hot pressing of a rolled alloy sheet.
Prior Arts to cope with these problems occurred in a Ni-Fe alloy have been proposed:
(1) Japanese Patent Examined Publication No. 60-7017 discloses that a ferromagnetic Ni-Fe alloy consisting essentially of 75.0 to 84.9 wt. % Ni, 0.5 to 5.0 wt. % Ti, 0.0010 to 0.0020 wt. % Mg, and balance being Fe and inevitable impurities, the content of C and S as the inevitable impurities being C: 0.03 wt. % or less and S: 0.003 wt. % or less (hereafter referred to as "the prior art 1").
(2) Japanese Patent unexamined publication No. 62-227054 discloses a ferromagnetic Ni-Fe alloy consisting essentially of 70 to 85wt. % Ni, 1.2wt. % or less Mn, 1.0 to 6.0 wt. % Mo, 1.0 to 6.0 wt. % Cu, 1.0 to 5.0 wt. % Cr, 0.0020 to 0.0150 wt. % B, and balance being Fe and inevitable impurities, the content of S, P, and C as the inevitable impurities being 0.005 wt. % or less S, 0.01 wt. % or less P, and 0.01 wt. % or less C, and the weight ratio of the content of B to the content of the sum of S, P, and C being 0.08 to 7.0 (hereafter referred to as "the prior an 2" hereafter).
As described above, PC Permalloy has a feature of high magnetic permeability and weak coercive force. PC Permalloys which have been brought into practical use include 80% Ni-5% Mo-Fe (Supermalloy) and 77% Ni-5% Cu-4% Mo-Fe (Mo, Cu Pennalloy), and they give 150,000 of the initial magnetic permeability and 300,000 of the maximum magnetic permeability as ordinary level.
Recent development of electronics technology demands higher level than described above to utilize miniaturized high performance devices. To cope with the demand, the prior art 2 was introduced as a technology which improves the magnetic characteristics by the reduction of impurities and the addition of Cr.
Those prior arts have, however, problems described below.
The characteristics of the prior art 1 is to improve the hot workability through the fixation of S, an impurity element, by Mg which has a strong tendency to form sulfide. As disclosed in the embodiment, the alloy of the prior art 1 shows, however, a low reduction ratio to a level of 50 to 60% at a temperature range of 950.degree. to 1150.degree. C. which is a particularly important range in industrial processing. As a result, a hot working on the surface of such an alloy induces lots of defects on the slab surface.
The reduction ratio of area described above is defined as the ratio of the difference between the original cross sectional area A of a specimen and the minimum cross sectional area A' at break under a tensile stress at 1 s.sup.-1 or more strain rate being represented by the formula of [(A-A')/A.times.100] as percentage to the original cross sectional area. The value is measured using a tensile tester to break a specimen.
The characteristic of the prior art 2 is to improve the hot workability of an alloy through the reduction of the content of S, P, and C as impurities and through the addition of B to suppress the segregation of impurity elements to the grain boundaries. According to the experiments carried out by the inventors, however, the alloy of the prior art 2 was found to be extremely inferior in the hot workability. That is to say, the inventors prepared an ingot by melting the alloy No. 5 described in the example of the prior art 2 in a vacuum melting furnace, and cut the ingot to form a specimen of 5 mm diameter and 100 mm length from the prepared ingot. After heating the specimen to 1200.degree. C., it was cooled to 950.degree. C., the reduction ratio of the specimen was determined. The value was 35%.
Consequently, also the alloy of the prior art 2 gives a low reduction ratio at 950.degree. C. level which is an important range in hot working. As a result, when the alloy is subjected to hot working, the obtained slab has lots of surface defects.
Regarding the direct current magnetic characteristics, the reduction of impurities and addition of Cr, which are features of the prior art 2, gave 100,000 level of the initial magnetic permeability at the maximum immediately after the final annealing (1100.degree. C..times.3hrs) in hydrogen atmosphere. So the art can not respond to applications which request higher magnetic characteristics.
Also in the prior art 1, the direct current magnetic permeability immediately after the final annealing (1100.degree. C..times.3 hrs) in hydrogen atmosphere gave only 26,000 level of the initial magnetic permeability. So the art also can not cope satisfactorily with the applications which request higher magnetic characteristics.